Hydrodynamic Numerical Model Of WEC With An Airbag And Analysis Of Absorption Efficiency

As a kind of renewable energy,wave energy has the characteristics of wide distribution,high energy density,and convenient development and utilization.In order to use wave energy effectively,scholars around the world have proposed several representative wave energy conversion devices(WEC),which can be divided into oscillating water columns(OWC)WEC,point absorption WEC,pendulum WEC et.al according to the working principle.Most of the part for wave energy extraction in above device can be regarded as rigid structure.So high working efficiency can only occur in a narrow wave frequency range when the structure parameters are determined.The wave spectrum is wide in actual sea conditions.In order to increase the wave frequency width with the high energy convention efficiency,a WEC with a floating air bag was proposed by Chaplin et.al(2015).The device includes an upper compressible airbag and a sinker attached to the bottom of the airbag.PTO is arranged between them and the three parts form a closed system.The airbag expands and contracts under the action of wave,converting the wave energy to electricity by forced the air to flow through the power take-off(PTO).Because of the particularity of the airbag,the static analysis method,the dynamic analysis method and the wave energy conversion method of the device are established.The iterative method for the equilibrium shape of the airbag under different parameters is established according to the static balance.The correctness of the static analysis model is verified by comparison with the experimental results of Chaplin et.al.In hydrodynamic model,the radiation wave force caused by the deformation of the airbag is neglected,which means the wave excitation force on the airbag is equal to the corresponding wave force on the rigid airbag when the airbag is stationary.The excitation force is solved based on the linear potential flow theory by boundary element method.The relationship between air pressure and volume deformation under the wave force is established based on the principle of gas isentropic.The inertia force of the airbag is neglected in the dynamic balance equation,that is,the device is in a static equilibrium state under the wave action in every moment.Finally,the method of wave energy absorption efficiency is established according to the iterative method for the static equilibrium shape of the airbag and the gas isentropic principle.The static analysis is carried out for floating airbags and sinker fixed airbags respectively.The effects of parameters such as internal air pressure,ballast weight and tendon length on the stiffness of the airbag were studied.The influence of different structural parameters on the wave energy conversion efficiency of the WEC with a fixed airbag is studied by the control variable method.The trend and physical meaning of each parameter are analyzed,and the optimal structural parameters are obtained according to the study.